Inline pipeline inspection involves sending an inspection tool through a pipe, typically while the pipe is carrying product. The inspection tools are commonly referred to as “pigs” and the process of sending a pig through a pipe is known in the industry as “pigging”. The pigs often travel through the pipeline for long distances, perhaps several hundred miles, and are propelled by the movement of the product (either liquid or gas) in the pipeline. Typically, to provide the motive force to move the pig, polyurethane sealing cups, or disks, are integrated into the pig's construction. The cups may completely seal or semi-seal at or against the inner wall of the pipe, creating a pressure differential that powers or propels the pig through the pipe.
As a matter of safety and logistics, pipeline operators must exercise great care with respect to the location, status or movement of a pig while the pig is in the pipeline. A lost pig could damage the system or cause an expensive shutdown. To prevent these problems, operators carefully guide the pig by opening and closing valves to direct the pig past pumping stations and “un-piggable” segments of the pipeline. In order to guide the pig, the pipeline operators must know the location of the pig and know when it has passed specific landmarks or above ground references (AGRs). During a pigging operation, tracking teams must monitor specific points along the pipeline to determine when the pig passes the landmark. This often requires coordination between multiple teams located miles apart. Often, operations are conducted in remote areas that lack communications infrastructure such as cellular telephone towers.
Many instruments are currently in use to detect the passage of inspection pigs as they travel through pipelines. Typically, pigs exhibit a variety of characteristics during use that make them possible to detect as they pass underground through a pipeline. For example, many pigs use magnets as a part of the inspection process. The magnets generate a magnetic field that is detectable above ground. Also, virtually all pigs create noise and vibrations as they move through the pipe. Sensitive geophones or accelerometers or seismographs (such as those used to detect earthquakes) can pick up these noises and allow tracking technicians to listen for the pig's approach and subsequent passage. Further, many pigs carry an onboard transmitter that emits a low frequency electromagnetic field (typically in the 15 to 22 Hz range). Electromagnetic fields in these frequencies can pass through metal pipes and earth and can be detected above the surface with the proper equipment.
However, each of these detection methods has shortcomings and no method is always reliable. Further, none of these detection methods is suitable for all pigs because different pigs have different characteristics. For example, a foam pig is not detectable with magnetic sensing and a pig without a low frequency transmitter obviously needs to be detected by something other than a low frequency receiver. Different soil conditions may also hinder or interfere with one or more detections methods. Electrical transmission lines can also severely hinder low frequency detection and highway noises can obscure noises normally detected with a geophone.
To overcome some of these barriers, pigging crews use multiple devices with different detection methods in order to improve the likelihood of detecting the pig as it passes the AGM. However, using multiple devices increases the complexity and cost to the system and also increases the workload for the pigging crew. In some cases, pigging crews have relied on a single AGM and mistakenly deployed the wrong type of AGM and failed to detect the pig as it went by.
A common application of pigs in the pipeline industry is the use of magnetic flux leakage (MFL) tools for the detection of thin or weakened areas or other anomalies in the wall of the pipe. Precisely locating an anomaly requires an above-ground marker at a known position to corroborate the positional information gathered by the inspection tool. As a pig logs the inspection data, the pig's processor records the time and the cumulative distance as measured by the pig's odometer. However, odometers sometimes slip, or accumulate pipeline deposits on the odometer wheel, thus changing the diameter of the odometer wheel and thus leading to inaccurate measurements. These problems can be the source of significant cumulative error over long distances. The above-ground marker detects the pig as it passes known reference points and provides benchmarks that can be used to correct for cumulative odometer errors. After the pig is retrieved from the pipeline, the time of the pig's passage past known benchmarks (as a result of the tracking crew detecting the pig at a known time and a known place) is compared with the corresponding timestamps made from the pig's internal time clock as the pig passed the reference point. By comparing the two times, technicians are able to post-process the data to correct for any errors in the pig's odometer. Post-processing requires a technician, or log analyst, to use the time stamps, AGM position, and pipeline drawings to match logged pipe features to the corresponding features on the pipeline. Typically, with the information available, an analyst is able to correctly match the inspection log with the drawings on a joint-to-joint basis. Keeping each of the clocks in the system synchronized as closely as possible is essential for accurately determining the pig's position at each reference point.
Thus, a need also exists for system-wide clock synchronization. A need also exists for a simplified system that is capable of detecting pigs using a variety of detection methods to cover the widest possible range of pigs and operating conditions. A need also exists for devices with improved detection capability. Therefore, there is a need in the art for a pig tracking system that overcomes the shortcomings of the prior art.